In recent years, fiber-optic chemical sensors, sometimes called optrodes, have been developed to detect the presence and monitor the concentration of various analytes, including oxygen, carbon dioxide, and hydrogen ions (i.e., pH) in liquids and in gases. Such sensors are based on the recognized phenomenon that the absorbance, and in some cases, the luminescence, that is, the phosphorescence or fluorescence of certain indicator molecules, are specifically perturbed in the presence of specific analyte molecules. The perturbation of the luminescence and/or absorbance profile can be detected by monitoring radiation that is absorbed, reflected, or emitted by the indicator molecule in the presence of a specific analyte.
Fiber-optic sensors relying on these characteristics position the analyte sensitive indicator molecule in a light path at a desired measurement site. Typically, the optical fiber transmits electromagnetic radiation from a light source to the indicator molecule, and the reflectance from or absorption of light by the indicator molecule gives an indication of the gaseous or ionic concentration of the analyte. Alternatively, for monitoring other analytes such as oxygen, the optical fiber transmits electromagnetic radiation to the indicator molecule, exciting it into a type of luminescence, for instance phosphorescence, and the level and/or duration of phosphorescence by the indicator molecule serves as an indication of the concentration of that gas in the surrounding fluid. In the prior art sensors, the indicator molecules are typically disposed in a sealed chamber at the distal end of an optical fiber, and the chamber walls are permeable to the analytes of interest.
One problem with the known sensing systems of the type described is that the optical fiber and chamber attached to the end of the probe are prone to physical damage. The optical fibers with attached sensing chambers are delicate because they are disposed as an external appendage at the end of a probe, extending distally beyond the catheter through which the probe is positioned inside a patient's circulatory system or other physiological features. Any mishandling of the catheter can easily result in damage to the delicate sensor chamber. An additional problem with the known sensing systems described above is that the structure of the chambers and probe configuration often encourage the formation of blood clots or thrombi. Typically, the probes of the prior art comprise discreet optical fibers for each blood gas parameter such as oxygen, carbon dioxide, and pH. This multiplicity of fibers adds to the diameter of the complete probe and provides interfiber crevices that encourage thrombi formation. Furthermore, the complexity and difficulty of manufacturing multifiber probes is well known, due to the small diameters of the fibers and requirements of their arrangement.
Even though a bundled optical fiber probe for sensing a plurality of analytes may have a remarkably small overall cross section, its size can still preclude use in neonatal or pediatric applications in which the patient's veins or arteries are too small in diameter for insertion of the probe assembly. Other problems with known sensing systems relate to the difficulty in reliably placing the sensing end in the environment to be sampled and thereafter continually monitoring the position of the sensor. Correct initial placement of the sensor and maintenance of the initial placement is important in order to obtain reliable results. If the sensor is not correctly placed, the results obtained can be misleading. Correct placement of the sensor is particularly important in the arena of neonatal applications where the presence of analytes such as oxygen, carbon dioxide, and hydrogen ions in a fetus are monitored as a means of evaluating the fetal condition. Prior art multianalyte sensors have failed to effectively deal with several of the problems set forth above.